Embodiments of the present invention relate to solid state batteries, and especially to thin film batteries, and their fabrication.
Solid state batteries, which are batteries that are absent liquid and in the solid state, such as for example, thin film batteries, are being rapidly developed for many applications. The energy density and specific energy of a battery, which expresses the energy capacity of the battery per unit volume and weight, respectively, are important performance measures. Generally, solid state and thin film batteries provide higher energy density and specific energy than liquid containing batteries. In small sizes, solid state batteries are often fabricated by microelectronic processing techniques, and used in applications such as for example, portable electronics, medical devices, and space systems. In larger sizes, the batteries can be used to power electric cars or store electrical power in a home or electrical grid.
A solid state battery can have a one or more battery cells connected in series or parallel within the battery. Each battery cell comprises battery components such as electrodes like the anode, cathode, anode current collector, cathode current collector, and an electrolyte between the electrodes. However, the solid state battery components are often sensitive to exposure to the surrounding external environment, for example air, oxygen, carbon monoxide, carbon dioxide, nitrogen, moisture and organic solvents. Thus, protective packaging for a battery or a battery cell within the battery is used to reduce or eliminate exposure of the thin films to the external environment. For example, a protective sheet of polymer can be laminated onto the battery structure to serve as protective packaging. However, such conventional packaging structures are often thicker than the original battery. For example, in thin film batteries, the laminated sheets typically have to be tens or hundreds of micrometers thick to provide adequate protection and structural support, whereas the battery component themselves are only a few micrometers thick. Thus, the laminated packaging substantially increases the weight and volume of the thin film battery, and consequently, reduces its energy density and specific energy.
A protective covering film deposited onto the battery structure can also serve as protective packaging. Such protective films can include ceramics, metals, and polymer materials. However, such films often do not provide protection from the elements for a long time, and eventually allow gases or other atmospheric elements to leach through the defects in the films in a few months. The covering films also do not provide adequate structural support, and their use may entail additional packaging to increase the structural strength of the battery, which further reduces the battery energy density. Furthermore, these films often have to also be several tens of micrometers thick to provide adequate environmental protection, and this additional thickness still further limits energy density.
A sheet of glass can also be positioned over the battery component films to serve as protective packaging. However, the glass sheet presents an inflexible boundary to the underlying battery component films. For example, the anode typically expands and contracts during the charge and discharge cycles of the battery. The inflexible glass sheet restricts such expansion creating mechanical stresses in the anode which may eventually lead to mechanical or chemical failure and reduce the lifetime or degrade the performance of the battery. The glass sheet is also typically thick and weighty, thus further reducing the energy density and specific energy of the battery.
For reasons including these and other deficiencies, and despite the development of various protective packaging structures for solid state and thin film batteries, further improvements in protective thin battery packaging and methods of fabrication are continuously being sought.